Defining a reference range of acetabular inclination and center-edge angle of the hip in asymptomatic individuals

Acetabular morphology is an important predictor of the severity of osteoarthrosis and survival of hip prostheses but there is limited data on the normal range of acetabular measurements on plain radiographs. The aim of this project was to determine the statistically normal ranges of acetabular inclination (AI) and center-edge angle (CEA).

One hundred coronal CT localizers (50 men and 50 women aged 20-30 years) were included in this study. All the patients underwent CT examination for thoracic or intra-abdominal indications. Patients with pelvic disease, fractures, history of serious trauma, or previous pelvic surgery were excluded. One pair of independent observers measured the AI and pelvic tilt (PT), and a further pair measured the center-edge angle (CEA), using electronic calipers on a high-resolution PACS workstation.

AI and CEA measurements were obtained for 200 hips. There was very good intra-class correlation between the observers (r = 0.7-0.8). The mean AI was 38.8° (2SD 32.1-45.5°). That in men was 38.0° (2 SD 31.8-44.1°) and 39.6° (2 SD 32.7-46.8°) in women, which was statistically significantly different (p < 0.001). The mean CEA measurement for all patients was 36.3° (SD 13.8°), for men 37.7° (SD 10.8°) and for women 34.9° (SD 11.4°) with a statistically significant gender difference (p < 0.001). The mean pelvic tilt measurement (sacro-coccygeal-pubic symphysis) was 38.3 mm (2 SD 18.3-58.3 mm) with a significant gender difference (p < 0.001).

The results of this study define reference ranges of two common measures of acetabular morphology and confirm statistically significant differences between men and women.